The invention relates to a method for the generation of a frequency-variable output voltage ua by means of a modular multilevel power converter (M2C), wherein through the switching on and off of sub-modules, the power converter generates a sinusoidal alternating voltage, approximated by discrete voltage levels, with a first angular frequency ω0, which lies between a zero frequency ω0,min and a second angular frequency ω0, max, from an input voltage Ud.
The invention also relates to an arrangement for performing the method for the generation of a frequency-variable output voltage ua from an input voltage Ud by means of a modular multilevel power converter (M2C), with which branches are arranged for the generation of an output voltage, each comprising a series connection of a number of sub-modules.
Multilevel power converters can generate voltages at their output terminals with a low harmonic distortion, which come very close to a desired sinusoidal curve of the voltage.
At present, many multilevel topologies are disclosed, such as 3-level NPC, Flying Capacitor Converters, Cascaded H-Bridge Converters, as in Bin Wu, “High-Power Converters and AC Drives”, John Wiley & Sons Inc., Hoboken, N.J., USA, 2006.
A relatively new topology is the modular multilevel power converter (M2C, M2LC), as described in R. Marquardt, A. Lesnicar, J. Hildinger, “Modulares Stromrichterkonzept für Netzkupplungsanwendung bei hohen Spannungen” [“Modular power converter concept for use in network coupling at high voltages”], ETG-Fachtagung [specialist conference], Bad Nauheim, 2002 and in R. Marquardt, A. Lesnicar, “New Concept for High Voltage Modular Multilevel Converter”, Proc. Of IEEE Power Electronics Specialists Conference (PESC), Aachen, 2004.
This is at present the subject matter of application and research. An essential feature of the M2C is the series connection of many cells, the so-called sub-modules.
It is necessary for the mode of operation for each of these cells to have an energy storage device, which can take the form of a direct voltage capacitor.
During the operation of multilevel power converters according to the prior art the following conditions should be observed:
Since the energy storage devices are not charged or discharged by external wiring networks, such as galvanically isolated rectifiers, the change over time of the stored energy is heavily dependent on the operation of the converter and/or the load. Thus according to [R. Marquardt, . . . “Modulares Stromrichterkonzept für Netzkupplungsanwendung bei hohen Spannungen” (Modular power converter concept for use in network coupling at high voltages)] the design-relevant energy fluctuation during so-called “circulating-current-free operation” depends indirectly proportionally on the fundamental frequency of the load current.
With circulating currents, current components are named here in the branches of the converter which flow neither via the connection to the DC intermediate circuit, nor via the load. The “circulating-current-free” operation thus indicates that the currents in the branches of the converter only contain components which flow via the DC intermediate circuit and the load.
According to this, a “circulating-current-free operation” at “low” fundamental frequencies is either not possible or only possible with restrictions. This should be noted, in particular, with respect to the operation of electrical machines.
In particular, in the case of motor or generator applications, a very fast startup of the machine to the design frequency fN has to be achieved in order to restrict the energy input into the sub-module capacitors. Consequently other restrictions to the scope of application are incurred on the basis of the properties of the driven mechanical system.
Restriction of the operating frequency f0 of the converter to f0, Min<f0<f0, Max. With machine applications this implies a “hard” startup of the machine from the idling state.
One possibility of achieving the fast startup is to limit the operating frequency of the converter to 0<f0,Min<f0<=f0, Max. In particular, this indicates in the case of asynchronous machines a “hard” startup from the idling state to:
      f    Mech    =                    f                  0          ,                      Mi            ⁢                                                  ⁢            n                              p        *          (              1        -                  s          x                    )      where p is the number of pole pairs, fMech is the mechanical speed of the machine and sx is the slip.
Therefore no stationary operation is possible at frequencies f0<f0,Min, thus no direct current brakes and no soft start are possible from an idling state. It must be noted that a downstream mechanical system is not damaged as a result of the possible occurrence of torque impulses.
A further option to reduce the energy fluctuations is to match the amplitude of the load current as a function of the frequency f0, ÎL=g(f0), with the aim of restricting the energy input in the sub-module capacitors. This leads to a reduction in the power of the M2C; in the case of drives this corresponds to a reduction in the torque. In the case of network applications this corresponds to a reduction in the power to be transferred.
Further, the influence of the energy input into the sub-module capacitors due to additional current components in the converter, which do not flow via the DC connection, or the load, must be taken into account. Frequently the consequences of this influence are additional switching and conduction losses in the semiconductor devices, additional losses within the converter at the ohmic resistances, increase in the silicon surface of the semiconductor devices, as well as a reduction in the efficiency.
An influencing of the energy input into the capacitors of the sub-modules through a variation of the difference from the sum of the terminal voltage of the sub-modules above the connection terminals of the load or of the mains and the sum of the terminal voltage of the sub-modules below. This variation corresponds to a modulation of the common mode voltage
This measure is not possible on all systems. If possible, the modulation of the medium voltage level control is often restricted by the technical requirements of the application. Examples of this are insulation requirements. The modulation of the medium voltage level control of the sub-modules leads to a change in the switching losses.
An operation of the power converter according to [Korn, Winkelnkemper, “Low output frequency . . . ”] is also possible with f0<<fN by suitably combining the influencing of the energy input by circulating currents and varying the common mode voltage. In particular, an operation with fa)=0 Hz is possible with the cited method.
Explanation of this are found in A. J. Korn, M. Winkelnkemper, and P. Steimer, “Low output frequency operation of the modular multi-level converter”, in Energy Conversion Congress and Exposition (ECCE), 2010 IEEE, September 2010, pp. 3993, 3997.
A further possibility is also the layout of the sub-module capacitors on a minimal continuous operation frequency f0, Min. An over-dimensioning with respect to the nominal frequency fN, increases the costs. Also, the additional stored energy in the converter increases the cost required to satisfy the safety requirements.
In summary the following applies:                The operation of a modular multilevel power converter (M2C) for f0<fN at the rated current of the converter leads to increasing expense in terms of capacitors and/or power semiconductors and/or additional requirements of the load, for example relating to the insulation of the neutral point of a machine.        The operation for f0<fN at a reduced current clearly restricts the characteristics of the converter, such that possible areas of application are drastically reduced. Thus, for example, a 4-quadrant drive, in which M=MN is required for the torque of the machine over the whole range, is not possible.        